The Cause of Anomalous Potassium-Argon “Ages” for Recent Andesite Flows at Mt. Ngauruhoe, New Zealand, and the Implications for Potassium-Argon “Dating”

Abstract

New Zealand’s newest and most active volcano, Mt. Ngauruhoe in the Taupo Volcanic Zone,
produced andesite flows in 1949 and 1954 and avalanche deposits in 1975. Potassium-argon “dating”
of five of these flows and deposits yielded K-Ar model “ages” from <0.27 Ma to 3.5±0.2 Ma. “Dates” could
not be reproduced, even from splits of the same samples from the same flow, the explanation being
variations in excess 40Ar* content. A survey of anomalous K-Ar “dates” indicates they are common,
particularly in basalts, xenoliths, and xenocrysts such as diamonds that are regarded as coming from
the upper mantle. In fact, it is now well established that there are large quantities of excess 40Ar* in the
mantle, which in part represent primordial argon not produced by in situ radioactive decay of 40K and
not yet outgassed. And there are mantle–crust domains between, and within which, argon circulates
during global tectonic processes, magma genesis, and mixing of crustal materials. This has significant
implications for the validity of K-Ar and 40Ar/39Ar “dating.”

This paper was originally published in the Proceedings of the Fourth International Conference on Creationism, pp. 503–525
(1998), and is reproduced here with the permission of the Creation Science Fellowship of Pittsburgh (www.csfpittsburgh.org).

Introduction

Mt. Ngauruhoe is an andesite stratovolcano of 2291 m elevation, rising above the Tongariro volcanic massif
within the Tongariro Volcanic Center of the Taupo Volcanic Zone, North Island, New Zealand (fig. 1).1, 2 Though
not as well publicized as its neighbor, Mt. Ruapehu (about 12 km to the south), Ngauruhoe is an imposing,
almost perfect cone that rises more than 1000 m above the surrounding landscape. Eruptions from a central
400 m diameter crater have constructed the steep (33°) outer slopes of the cone.3, 4

Geologic Setting

The Taupo Volcanic Zone, a volcanic arc and marginal basin of the Taupo-Hikurangi arc-trench (subduction)
system,5 is a southward extension on the Tonga-Kermadec arc into the continental crustal environment of New
Zealand’s North Island. It has been interpreted as oblique subduction of the Pacific plate beneath the Australian
plate. The zone extends approximately 300 km north-northeast across the North Island from Ohakune to White
Island (fig. 1) and is up to 50 km wide in the central part, narrowing northward and southward. This volcanotectonic
depression (Taupo-Rotorua depression6) comprises four rhyolitic centers (Rotorua, Okataina, Maroa,
and Taupo), plus the calc-alkaline Tongariro Volcanic Center, part of a young (<0.25 Ma) andesite-dacite volcanic
arc with no associated rhyolitic volcanism extending along the eastern side of the zone.7

The Tongariro Volcanic Center extends for 65 km south-southwest from Lake Taupo at the southern end
of the Taupo Volcanic Zone (fig. 1) and consists of four large predominantly andesite volcanoes (Kakaramea,
Pihanga, Tongariro, and Ruapehu; fig. 2); two smaller eroded centers at Maungakatote and Hauhungatahi; a
satellite cone and associated flows at Pukeonake; and four craters at Ohakune (fig. 2).8, 9

Fig. 1. The location of Mt. Ngauruhoe in the Taupo Volcanic
Zone (TVZ), New Zealand, showing the main structural
features. The shaded area is the andesite arc, and the inset
shows the major components of the boundary between the
Australian and Pacific Plates in the New Zealand region
(arrows indicate relative motions). Solid triangles are
basalt-andesite volcanoes.15

Most vents lie close to the axis of a large graben in which Quaternary volcanic rocks overlie a basement of
Mesozoic greywacke and Tertiary sediments.10, 11 North-northeast-trending normal faults with throws up to
30 m cut the volcanoes within the graben. Nearly all vents active within the last 10 ka lie on a gentle arc which
extends 25 km north-northeast from the Rangataua vent on the southern slopes of Ruapehu through Ruapehu
summit and north flank vents, Tama Lakes, Ngauruhoe, Red Crater, Blue Lake, and Te Mari craters. None
of the young vents lie on the mapped faults, which mostly downthrow toward the axis of the graben. The
vent lineation lies above this axis, which is considered to mark a major basement fracture that allows the intrusion of andesite dikes.12, 13, 14

The Tongariro volcanics unconformably overlie late
Miocene marine siltstones beneath Hauhungatahi, and
a minimum age for the onset of volcanism is measured
by the influx of andesite pebbles in early Pleistocene
conglomerates of the Wanganui Basin to the south.16, 17
The oldest dated lavas from the Tongariro massif
are hornblende andesites exposed at Tama Lakes
between Ngauruhoe and Ruapehu, at 0.26±0.003 Ma;
from Ruapehu, 0.23±0.006 Ma; and from Kakoramea,
0.22±0.001 Ma (potassium-argon dates).18

Tongariro itself is a large volcanic massif that
consists of at least twelve composite cones, the youngest
and most active of which is Ngauruhoe. A broad division
has been made into older (>20 ka) and younger (<20 ka)
lavas.19, 20 There is a north-northeast alignment of
the younger vents of Tongariro, particularly evident
between Te Mari and Ngauruhoe.

Ngauruhoe

Ngauruhoe is the newest cone of the Tongariro
massif and has been active for at least 2.5 ka.21, 22, 23 It has
been one of the most active volcanoes in New Zealand,
with more than seventy eruptive episodes since
1839, when the first steam eruption was recorded by
European settlers.24, 25, 26 Prior to European colonization
the Maoris witnessed many eruptions from the
mountain.27 The first lava eruption seen by European
settlers occurred between April and August 1870, with
two or three flows witnessed spilling down the northwestern
flanks of the volcano on July 7.28, 29 Following
that event there have been pyroclastic (ash) eruptions
every few years,30 with major explosive activity in
April–May 1948.

The next lava extrusion was in February 1949,
beginning suddenly with ejection of incandescent
blocks, and a series of hot block and ash flows down the
northwestern slopes on February 9.31, 32 The southern
sub-crater filled with lava, which by late on February
10 had flowed over the lowest part of the rim and down
the northwest slopes of the cone. By February 12 the
flow had ceased moving, subsequent mapping placing
its volume at about 575,000 m3 (fig. 3).33, 34 Further
explosive pyroclastic (ash) eruptions followed, reaching
a maximum about February 19-21 . The eruptions ended
on March 3.

The eruption from May 13, 1954, to March 10,
1955, began with explosive ejection of ash and blocks,
although red-hot lava had been seen in the crater five
months previous.35, 36 The eruption was remarkable
for the estimated large volume of almost 8 million m3 of
lava that then flowed from the crater from June through
September 1954, and was claimed to be the largest
flow of lava observed in New Zealand (that is, by the
European settlers).37, 38 The lava was actually expelled
from the crater in a series of seventeen distinct flows on
the following dates:39, 40

June 4, 30

July 8, 9, 10, 11, 13, 14, 23, 28, 29, 30

August 15 (?), 18

September 16, 18, 26

Fig. 3. Map of the northwestern slopes of Mt. Ngauruhoe
showing the lava flows of 1949 and 1954 and the 1975
avalanche deposits.48, 49, 50, 51, 52 The location of samples collected
for this study are marked.

Fig. 3 shows the distribution of those 1954 lava flows
that are still able to be distinguished on the northwestern
and western slopes of Ngauruhoe. All flows
were of aa lava (as was the February 1949 flow), typified
by rough, jagged, clinkery surfaces made up of blocks of
congealed lava. The lava flows were relatively viscous,
some being observed at close quarters slowly advancing
at a rate of about 20 cm per minute.43, 44, 45 The August 18
flow was more than 18 m thick and still warm almost
a year after being erupted. Intermittent explosive
eruptions and spectacular lava fountaining during June
and July 1954 built a spatter-and-cinder cone around
the south sub-crater, modifying the western summit of
the mountain. Activity decreased for two months after
the last of the lava flows on September 26, but increased
again during December 1954 and January 1955 with
lava fountaining and many highly explosive pyroclastic
(ash) eruptions. The last ash explosion was reported on
March 10, 1955, but red-hot lava remained in the crater until June 1955.46, 47

After the 1954–1955 eruption, Ngauruhoe steamed semi-continuously, with numerous small eruptions of
ash derived from comminuted vent debris. Incandescent ejecta were seen in January 1973, and ash erupted in
December 1973 contained juvenile glassy andesite shards.53 Cannon-like, highly explosive eruptions in January
and March 1974, the largest since 1954–1955, threw out large quantities of ash and incandescent blocks, one
of which was reported as weighing 3000 tonnes and thrown 100 m.54, 55 Pyroclastic avalanches flowed from the
base of large convecting eruption columns, down the west and north slopes of the cone, and the crater became
considerably shallower.56, 57

A series of similar but more violent explosions occurred on February 19, 1975, accompanied by clearly
visible atmospheric shock waves and condensation clouds.58, 59, 60, 61 Ash and blocks up to 30 m across were ejected
and scattered within a radius of 3 km from the summit. The series of nine cannon-like, individual eruptions
followed a 1.5 hour period of voluminous gas-streaming emission, which formed a convecting eruption plume
between 11 km and 13 km high.62, 63, 64 The explosions took place at 20–60 minute intervals for more than five
hours. Numerous pyroclastic avalanches were also generated by fallback from the continuous eruption column,
the avalanches consisting of a turbulent mixture of ash, bombs, and larger blocks which rolled swiftly down
Ngauruhoe’s sides at about 60 km per hour.65, 66 The deposits from these avalanches and the later explosions
accumulated as sheets of debris in the valley at the base of the cone, but did not extend beyond 2 km from the
summit. It is estimated that a minimum bulk volume of 3.4 million m3 of pyroclastic material was erupted in
the seven-hour eruption sequence on that day.51 Fig. 3 shows the location of these avalanche deposits.

There have been no eruptions since February 1975. A plume of steam or gas is still often seen above the
summit of the volcano, as powerful fumaroles in the bottom of the crater discharge hot gases. However, the
temperature of these fumaroles in the crater floor has steadily cooled significantly since 1979, suggesting that
the main vent is becoming blocked.

Sample Collection

Field work and collection of samples was undertaken in January 1996. The Ngauruhoe area was accessed
from State Highway 47 via Mangateopopo Road. From the parking area at the end of the road, the Mangateopopo
Valley walking trail was followed to the base of the Ngauruhoe cone, from where the darker-colored recent lava
flows were clearly visible and each one easily identified on the northwestern slopes against the lighter-colored
older portions of the cone (fig. 3).

Eleven 2–3 kg samples were collected: two each from the February 11, 1949, June 4, 1954, and July 14, 1954,
lava flows and from the February 19, 1975, avalanche deposits; and three from the June 30, 1954 lava flows.
The sample locations are marked on Fig. 3. Care was taken to ensure correct identification of each lava flow
and that the samples collected were representative of each flow and any variations in textures and phenocrysts
in the lavas.

Laboratory Work

All samples were sent first for sectioning one thin section from each sample for petrographic analysis.
A set of representative pieces from each sample (approximately 100 g) was then dispatched to the AMDEL
Laboratory in Adelaide, South Australia, for whole-rock major, minor, and trace element analyses. A second
representative set (50–100 g from each sample) was sent progressively to Geochron Laboratories in Cambridge
(Boston), Massachusetts, for whole-rock potassium-argon (K-Ar) dating—first a split from one sample from each
flow, then a split from the second sample from each flow after the first set of results was received, and finally,
the split from the third sample from the June 30, 1954, flow.

At the AMDEL Laboratory each sample was crushed and pulverized. Whole-rock analyses were undertaken
by total fusion of each powdered sample and then digesting them before ICP-OES for major and minor elements,
and ICP-MS for trace and rare earth elements. Fe was analyzed for amongst the major elements by ICP-OES
as Fe2O3 and reported accordingly, but separate analyses for Fe as FeO were also undertaken via wet chemistry
methods. The detection limit for all major element oxides was 0.01%. For minor and trace elements the detection
limits varied between 0.5 and 20 ppm, and for rare earth elements between 0.5 and 1 ppm.

The potassium and argon analyses were undertaken at Geochron Laboratories under the direction of Richard
Reesman, the K-Ar laboratory manager. No specific location or expected age information was supplied to the
laboratory. However, the samples were described as andesites that probably contained “low argon” and therefore
could be young, so as to ensure the laboratory took extra care with the analytical work.

Because the sample pieces were submitted as whole rocks, the K-Ar laboratory undertook the crushing and
pulverizing preparatory work. The concentrations of K2O (weight %) were then measured by the flame photometry
method, the reported values being the averages of two readings for each sample.67, 68 The 40K concentrations
(ppm) were calculated from the terrestrial isotopic abundance using the measured concentrations of K2O. The
concentrations in ppm of 40Ar*, the supposed “radiogenic” 40Ar, were derived using the conventional formula
from isotope dilution measurements on a mass spectrometer by correcting for the presence of atmospheric
argon whose isotopic composition is known.69 The reported concentrations of 40Ar* are the averages of two
values for each sample. The ratios 40Ar*/Total Ar and 40Ar/36Ar are also derived from measurements on the
mass spectrometer and are also the averages of
two values for each sample.

Table 1. Whole-rock, major-element oxide analyses of recent
lava flows at Mt. Ngauruhoe, New Zealand, as reported in the
literature.

Petrography and Chemistry

Clark reported that most of the flows from
Ngauruhoe are labradorite-pyroxene andesite
with phenocrysts of plagioclase (labradorite),
hypersthene, and rare augite in a hyalopilitic
(needle-like microlites set in a glassy mesostasis)
groundmass containing abundant magnetite.70
However, all lava, lapilli, and incandescent
blocks that have been analyzed from eruptions
this century also contain olivine; chemically they
may be classed as low-silica (or basaltic) andesites
(using the classification scheme of Gill).71 The
published analyses in Table 1 show only trivial
changes in composition between 1928 and 1975.
In fact, the 1954 and 1974 andesites are so similar
that Nairn, et al., suggested that a solid plug of
1954 andesite was heated to incandescence and
partially remobilized on top of a rising magma
column in 1974.72 This plug was disrupted and
blown from the vent as ejecta ranging in texture
from solid blocks, through expanded scoria to
spatter bombs.

Table 2 lists the whole-rock major element
analyses of the eleven samples collected in this
study. Comparison of the data for each flow with the corresponding data in Table 1 indicates that in their bulk
chemistries all the samples analyzed (and thus all the flows) are virtually identical to one another, the trivial
differences being attributable to the statistics of analytical errors, sampling, and natural variations. Thus it is
not unreasonable to conclude that these basaltic andesites are cogenetic, coming from the same magma and
magma chamber, even as they have been observed to flow from the same volcano.
Nevertheless, Nairn, et al., suggested that even though the 1949 and 1954 lavas were both olivine-bearing
andesite, the chemical analyses (table 1) showed the 1954 lava to be slightly more basic than the 1949 lava,
with slightly higher MgO, CaO, and total iron oxides, but lower SiO2 and alkalis.89 However, these trends are not
duplicated with any statistical significance by the analytical results of this study (table 2). At least, they found
that their analyses of the 1974 lava blocks and bombs were identical within the limits of error with the 1954
lava (table 1), which was also substantiated in this study with respect to the 1975 avalanche material and the
1954 lava (table 2).

Clark and Cole recognized five lava types in the Tongariro Volcanic Center based on the modal proportions
of the phenocryst minerals.90, 91 Graham modified this scheme to six types based on a combination of mineralogy and
chemistry, but given their uniform bulk chemistry and petrology, these Ngauruhoe lava flows group together as
plagioclase-pyroxene andesite within Graham’s “Type 1.”92 Cole, et al., have described Type 1 lavas as volumetrically
dominant within the Tongariro Volcanic Center and as exhibiting coherent chemical trends with increasing silica
content.93 They are relatively Fe-rich and follow a typical calc-alkaline trend on the AFM diagram.

Adapting the terminology of Gill, the Ngauruhoe lavas are described as basic andesites (53–58 wt% SiO2).94, 95
Their designation as plagioclase-pyroxene andesites is
based on the predominant phenocrysts present, with
plagioclase greater than or equal to pyroxene. Two
modal analyses are listed in Table 3 which very closely
resemble the samples collected for this study.

All samples of the five lava flows examined in this
study exhibited a porphyritic texture, with phenocrysts
(up to 3 mm across) consistently amounting to
35–40% by volume. The phenocryst assemblage is
dominated (2:1) by plagioclase, but orthopyroxene and
augite (clinopyroxene) are always major components,
while olivine and magnetite are only present in trace
amounts. This POAM phenocryst assemblage is a
typical anhydrous mineralogy.98 The groundmass
consists of microlites of plagioclase, orthopyroxene, and clinopyroxene, and is crowded with minute granules
of magnetite and/or Fe-Ti oxides. Small amounts (9–10%) of brown transparent (acid-residuum) glass are also
present, and the overall texture is generally pilotaxitic.

Table 3. Modal analyses of two recent lava flows at Mt.
Ngauruhoe, New Zealand, as reported in the literature.

Steiner stressed that xenoliths are a common constituent of the 1954 Ngauruhoe lava, but also noted that
Battey reported the 1949 Ngauruhoe lava was rich in xenoliths. All samples in this study contained xenoliths,
including those from the 1975 avalanche material.99, 100 However, many of these aggregates are more accurately
described as glomerocrysts and mafic (gabbro, websterite) nodules.101 They are 3–5 mm across, generally have
hypidiomorphic-granular textures, and consist of plagioclase, orthopyroxene, and clinopyroxene in varying
proportions, and very occasionally olivine. The true xenoliths are often rounded and invariably consist of fine
quartzose material. Steiner also described much larger xenoliths of quartzo-feldspathic composition and relic
gneissic structure.

The plagioclase phenocrysts have been reported as ranging in composition from An89 to An40 (andesine to
bytownite), but in Ngauruhoe lavas are usually labradorite (An68-55). They are subhedral and commonly exhibit
complex oscillatory zoning with an overall trend from calcic cores to sodic rims.102, 103, 104 Thin outer rims are
usually compositionally similar to groundmass microlites. Twinning and hourglass structures are common.

Orthopyroxene predominates (>2:1) over clinopyroxene. Subhedral-euhedral orthopyroxene is typically
pleochroic and sometimes zoned. Compositions range from Ca4 Mg74
Fe22 to Ca3 Mg47 Fe50, but representative
bulk and partial analyses of Ngauruhoe orthopyroxenes indicate a hypersthene composition predominates,
which is confirmed by optical determinations.105, 106, 107, 108, 109, 110, 111 Euhdral-subhedral clinopyroxene is typically twinned
and zoned, but compositions show a restricted range of Ca43 Mg47 Fe10 to about Ca35 Mg40 Fe25, all of which is
augite.112, 113

The olivine present is strongly magnesian, analyses indicating some compositional zoning from Fo88 to Fo78.
The magnetite present in the groundmass is titanomagnetite, judging from the amount of TiO2 present in whole-rock
analyses (tables 1 and 2), but some ilmenite is likely to occur sporadically in association with it.114, 115

K-Ar Results

All analytical results received from Geochron Laboratories are listed in table 4, grouped in chronological
order according to the historic date of each flow. The 40Ar* quantity refers to the amount of radiogenic 40Ar
measured in each sample. All other quantities are self-explanatory, some of them being calculated from the
analytical results supplied by the laboratory.

The “age” of each sample is calculated from the analytical results using the general model-age
equation:116, 117

(1)

where:

t

= the “age”

?

= the decay constant of the parent isotope

Dt

= the number of daughter atoms in the rock presently

Do

= the number of daughter atoms initially in the rock

Pt

= the number of parent atoms presently in the rock

To date a rock, Dt and Pt are measured, and equation (1) can then be used if an assumption about the original
quantity of daughter atoms (Do) is made. Applied specifically to K-Ar dating, equation (1) thus becomes:

(2)

where:

t

= the “age” in Ma (millions of years)

5.543 × 10-10

= the current estimate for the decay constant of 40K

0.1048

= the estimated fraction of 40K decays producing 40Ar

40Ar*/40K

= the calculated mole ratio of radiogenic 40Ar to 40K in the sample

It should be noted that to make equation (2) equivalent to equation (1), 40Ar* is assumed to be equal to (Dt - Do),
which thus means the 40Ar* measurement has included within it an assumption concerning the initial quantity
of 40Ar in the rock, namely, no radiogenic argon is supposed to have existed when the rock formed (that is,
Do = 0). Thus equation (2) yields a “model age” assuming zero radiogenic argon in the rock when it formed.

The model ages listed in Table 4 range from <0.27 Ma to 3.5±0.2 Ma. However, it should be noted that the
samples, one from each flow, that yielded model ages of <0.27 Ma and <0.29 Ma (that is, below the detection
limits of the equipment for 40Ar*) were all processed at the K-Ar laboratory in the same batch, suggesting
the possibility of a systematic problem with the analytical procedure and equipment (in particular, the gas
extraction “line”). When this question was raised with the laboratory manager, Richard Reesman, he kindly
rechecked his equipment and then re-ran several of the samples, producing similar results and thus ruling out
a systematic laboratory “error.”

However, an independent blind check was then made, by submitting to the K-Ar laboratory duplicate splits
from two samples already analyzed, to establish if results really were reproducible. The samples chosen were
the A and B samples of the June 30, 1954, flow, because their first splits had produced the lowest and highest
model ages, <0.27 Ma and 3.5±0.2 Ma, respectively. The results of these additional analyses are shown in Table
4 as A#2 and B#2, and yielded model ages of 1.3±0.3 Ma and 0.8±0.2 Ma, respectively. Clearly, reproducibility
was not obtained, but this is not surprising given the analytical uncertainties at such low to negligible levels of
40Ar*, which are at the detection limits of the laboratory’s equipment (R. Reesman, pers. commun., February 19,
1997, and January 26, 1998).

Discussion

In spite of the wide variations in model “ages” obtained between and within these recent lava flows, and of
the difficulties obtaining analytical reproducibility, it is apparent that the cause of the anomalous K-Ar model
“ages” is excess argon in the lavas, that is, non-zero concentrations of radiogenic argon (40Ar*). This of course
is contrary to the assumption of zero radiogenic argon in equation (2) for calculating the model “ages.” When
analyzed the oldest of the lavas was less than 50 years old, so there has been insufficient time since cooling for
measurable quantities of 40Ar* to have accumulated within the lavas due to the slow radioactive decay of 40K.
Thus the measurable 40Ar* can’t be from in situ radioactive decay since cooling, and therefore must have been
present in the molten lavas when extruded from Mt. Ngauruhoe.

No Radiogenic Argon Assumption Violated by Many Anomalous “Ages”

The assumption of no radiogenic argon (40Ar*) when the rocks formed is usually stated as self-evident. For
example, Geyh and Schleicher state:

What is special about the K-Ar method is that the daughter nuclide is a noble gas, which is not normally
incorporated into minerals and is not bound in the mineral in which it is found.118 (p. 56)

Similarly, Dalrymple and Lanphere state:

a silicate melt will not usually retain the 40Ar that is produced, and thus the potassium-argon clock is not “set”
until the mineral solidifies and cools sufficiently to allow the 40Ar to accumulate in the mineral lattice.119 (p. 46)

Dalrymple has recently put the argument more strongly:

The K-Ar method is the only decay scheme that can be used with little or no concern for the initial presence
of the daughter isotope. This is because 40Ar is an inert gas that does not combine chemically with any other
element and so escapes easily from rocks when they are heated. Thus, while a rock is molten the 40Ar formed by
decay of 40K escapes from the liquid.120 (p. 91)

However, these dogmatic statements by Dalrymple are inconsistent with even his own work on historic lava
flows, some of which he found had non-zero concentrations of 40Ar* in violation of this key assumption of the
K-Ar dating method.121 He does go on to admit, “Some cases of initial 40Ar remaining in rocks have been
documented but they are uncommon,” but then refers to his study of 26 historic, subaerial lava flows.122, 123 Five
(almost 20%) of those flows contained “excess argon,” but Dalrymple still then says “that ‘excess’ argon is rare
in these rocks!” The flows and their “ages” were:124

Hualalai basalt, Hawaii (AD 1800–1801)

1.6±0.16 Ma

1.41±0.08 Ma

Mt. Etna basalt, Sicily (122 BC)

0.25±0.08 Ma

Mt. Etna basalt, Sicily (AD 1792)

0.35±0.14 Ma

Mt. Lassen plagioclase, California (AD 1915)

0.11±0.03 Ma

Sunset Crater basalt, Arizona (AD 1064–1065)

0.27±0.09 Ma

0.25±0.15 Ma

Far from being rare, there are numerous examples reported in the literature of excess 40Ar* in recent or young volcanic rocks producing excessively old whole-rock K-Ar “ages”:

Other studies have also reported measurements of excess 40Ar* in lavas. Fisher investigated submarine
basalt from a Pacific seamount and found “the largest amounts of excess 4He and 40Ar ever recorded” (at that
time).142 McDougall not only found “extraneous radiogenic argon present in three of the groups of basalt flows”
on the young volcanic island of Réunion in the Indian Ocean, but “extraneous argon” was also “detected in
alkali feldspar and amphibole in hyperbyssal drusy syenites that are exposed in the eroded core of Piton des
Neiges volcano.”143 Significant quantities of excess 40Ar* have also been recorded in submarine basalts, basaltic
glasses and olivine phenocrysts from the currently active Hawaiian volcanoes, Loihi Seamount and Kilauea, as
well as on the flanks of Mauna Loa and Hualalai volcanoes, also part of the main island of Hawaii, and in
samples from the Mid-Atlantic Ridge, East Pacific Rise, Red Sea, Galapagos Islands, McDonald Seamount and
Manus Basin.144, 145, 146, 147 Patterson, Honda, and McDougall claimed that some of the initial Loihi analytical results
were due to atmospheric contamination of the magma either during intrusion or eruption, but subsequent
work has confirmed that the excess 40Ar* is not from atmospheric contamination at all.148, 149, 150

Excess 40Ar* Occluded in Minerals

Austin has investigated the 1986 dacite lava flow from the post–October 26, 1980, lava dome
within the Mount St. Helens crater and has established that the 10-year-old dacite yields a whole-rock
K-Ar model “age” of 0.35±0.05 Ma due to excess 40Ar* in the rock.151 He then produced concentrates of the constituent
minerals, which yielded anomalous K-Ar model “ages” of 0.34±0.06 Ma (plagioclase), 0.9±0.2 Ma (hornblende),
1.7±0.3 Ma (pyroxene), and 2.8±0.6 Ma (pyroxene ultra-concentrate). While these mineral concentrates were
not ultra-pure, given the fine-grained glass in the groundmass and some Fe-Ti oxides, it is nonetheless evident
that the excess 40Ar* responsible for the anomalous K-Ar “ages” is retained within the different constituent
minerals in different amounts. Furthermore, the whole-rock “age” is very similar to the “age” of the plagioclase
concentrate because plagioclase is the dominant constituent of the dacite.

That the excess 40Ar* can be occluded in the minerals within lava flows, rather than between the mineral
grains, has been established by others also. Laughlin, et al., found that the olivine, pyroxene, and plagioclase
in Quaternary basalts of the Zuni-Bandera volcanic field of New Mexico contained very significant quantities
of excess 40Ar*, as did the olivine and clinopyroxene phenocrysts in Quaternary flows from New Zealand
volcanoes.152, 153 Similarly, Poths, Healey, and Laughlin separated olivine and clinopyroxene phenocrysts from
young basalts from New Mexico and Nevada and then measured “ubiquitous excess argon” in them.154 Damon,
Laughlin, and Precious have reported several instances of phenocrysts with K/Ar “ages” 17 million years
greater than that of the whole rocks, and one K/Ar “date” on olivine phenocrysts of greater than 110 Ma in a
recent (<13,000 year old) basalt.155 Damon, et al., thus suggested that large phenocrysts in volcanic rocks contain the
excess 40Ar* because their size prevents them from completely degassing before the flows cool, but Dalrymple
concluded that there does not appear to be any correlation of excess 40Ar* with large phenocrysts or with any
other petrological or petrographic parameter.156

Most investigators have come to the obvious conclusion that the excess 40Ar* had to have been present
in the molten lavas when extruded, which then did not completely degas as they cooled, the excess 40Ar*
becoming “trapped” in the constituent minerals, and in some instances, the rock fabrics themselves. Laboratory
experiments have tested the solubility of argon in synthetic basalt melts and their constituent minerals near
1300°C at one atmosphere pressure in a gas stream containing argon.157, 158 When quenched, synthetic olivine in
the resultant material was found to contain 0.34 ppm 40Ar*. Broadhurst, et al., commented that “The solubility
of Ar in the minerals is surprisingly high,” and concluded that the argon is held primarily in lattice vacancy
defects within the minerals.159

In a different experiment, Karpinskaya, Ostrovskiy, and Shanin heated muscovite to 740–860°C under
high argon pressures (2800–5000 atmospheres) for periods of 3 to 10.5 hours.160 The muscovite absorbed
significant quantities of argon, producing K/Ar “ages” of up to 5 billion years, and the absorbed argon appeared
like ordinary radiogenic argon (40Ar*). Karpinskaya subsequently synthesized muscovite from a colloidal gel
under similar argon pressures and temperatures, the resultant muscovite retaining up to 0.5 wt% argon at
640°C and a vapor pressure of 4000 atmospheres.161 This is approximately 2,500 times as much argon as is found
in natural muscovite. These experiments show that under certain conditions argon can be incorporated into
minerals and rocks that are supposed to exclude argon when they crystallize.

Applications to the Mt. Ngauruhoe Andesite Flows

Therefore, the analytical results from the very recent (1949–1975) andesite flows at Mt. Ngauruhoe, New
Zealand, that yield anomalous K-Ar model “ages” because of excess 40Ar* are neither unique nor an artifact of
poor analytical equipment or technique. This realization that the presence of the excess 40Ar* in these rocks is
both real and measurable, and has not been derived from radioactive decay of 40K in situ, leads to the obvious
questions as to whether there is any pattern in the occurrences of excess 40Ar*, and from whence came this
excess 40Ar*?

It is clear that the excess 40Ar* was in the lavas when they flowed from the Mt. Ngauruhoe volcano and
were trapped in the andesite as it cooled. That there were gases in the lavas is readily evident from the copious
“frozen” bubble holes now in the rock, implying that much of the gas content escaped as the lavas flowed and
cooled. When choosing samples, care was taken to select pieces from each flow that were different from one
another (for example, copious “frozen” gas bubble holes compared with virtually no such holes). It is hardly
surprising, therefore, that the 40Ar* measurements on four of the five flows were consistent with such differences
the samples from each flow which had very few or virtually no “frozen” gas bubble holes yielded excess 40Ar* and
thus anomalous K-Ar model “ages,” whereas the other samples from each of these flows that contained copious
“frozen” gas bubble holes failed to yield detectable 40Ar* (<0.27 Ma and <0.29 Ma in Table 4).

The exception was the June 30, 1954, flow—not only was this expected relationship between excess 40Ar*
and lack of “frozen” gas bubble holes not duplicated, but analyses on duplicate splits off the same samples
yielded widely divergent results (<0.27 Ma versus 1.3±0.3 Ma and 3.5±0.2 Ma versus 0.8±0.2 Ma; see Table
4). Thus the presence (or absence) of excess 40Ar* must also depend on which portion of a rock sample is being
analyzed, which in turn implies dependence on the mineral constituents present, including the glass in the
groundmass. As already noted, Austin found widely different amounts of excess 40Ar* in the mineral separates
concentrated from Mount St. Helens 1986 dacite, while numerous other studies have located excess 40Ar*
in phenocrysts.162, 163, 164, 165, 166

Cooling Rates, Pressures, and Potassium Alteration

Another factor is the rate of cooling of lavas. Dalrymple and Moore found that the 1 cm thick glassy rim
of a pillow in a Kilauea submarine basalt had greater than forty times more excess 40Ar* than the basalt
interior just 10 cm below.167 The glassy pillow rim is, of course, produced by rapid quenching of the hot basalt lava
immediately as it contacts the cold ocean water, so the excess 40Ar* in the lava is rapidly trapped and retained.
Dymond obtained similar results on four deep-sea basalt pillows from near the axis of the East Pacific Rise.168
Dalrymple and Moore also found that the excess 40Ar* contents of the glassy rims of basalt pillows increased
systematically with water depth, leading them to conclude that the amount of excess 40Ar* is a direct function of
both the hydrostatic pressure and the rate of cooling.169 In a parallel study, Noble and Naughton reported K-Ar
“ages” from zero to 22 Ma with increasing sample depth for submarine basalts probably less than 200 years old,
also from the active Kilauea volcano.170

Seidemann has reported yet another intriguing relationship.171 He analyzed deep-sea basalt samples
obtained from DSDP drillholes in the floor of the Pacific Ocean basin and found that K-Ar “ages” increased with
increasing K contents of the basalts, a relationship he noted also appeared in similar data published by DSDP
staff (Seidemann172, fig. 1). In basalt pillows the K content increases from the margin to a maximum at an
intermediate distance into the pillows, whereas holocrystalline basalts show a decrease of K inward from the
margin.173 Seidemann concluded, as had others before him, that submarine weathering adds K to the basalts, as
does alteration at the time of formation, whereas the glassy pillow margins are largely impervious to seawater.
The net result, however, is unreliable K-Ar “dates,” because the measured 40Ar* was probably not derived
by radioactive decay of the measured 40K contents. Seidemann also determined that sediment cover is not a
significant barrier to the diffusion of K into basalt.

It is possible that some of these factors are relevant to the pattern of excess 40Ar* measured in the samples
from the recent Mt. Ngauruhoe andesite flows. For example, the surfaces of the flows would have cooled more
rapidly to crusts on top of the still molten flow interiors, which would certainly have been the case with the
August 18, 1954, flow that was reported as being 18 m thick. Furthermore, the overburden pressure within the
deep interiors of such thick flows would likewise inhibit degassing of the lava as it cooled. However, so long as
the crusts on the tops of the flows remained unbroken and intact they would have sealed in the molten lava
and its contained gases, including excess 40Ar*. But this sealing was probably short-lived, because today the
flows mostly outcrop as piles of pieces of andesite that look like rubble (typical aa lavas). The continued flow of
the lavas down the sides of the volcano would have broken up the crusts as soon as they congealed, as would
contraction with cooling, thus enabling the molten interiors to degas as they cooled. So the speed of cooling was
likely the most relevant factor, and this would have varied laterally and vertically within the flows, even at
localised scales of a few centimeters.

Any effects of weathering on the K contents of these flows can be discounted. On the one hand these
are subaerial flows that do not appear to have been subjected to leaching or addition of K, or to any
K-rich alteration for that matter, while on the other they have very uniform K contents (see tables 1, 2, and
4). This is not unexpected, given the fact they flowed from the same magma source/chamber close together
timewise. The K-Ar data in table 4 do not reveal any discernible relationship between K contents and
K-Ar model “ages” of these flows, unlike the negative correlation found in the Middle Proterozoic Cardenas
Basalt upper member flows in Grand Canyon, Arizona.174

Perhaps the key issues, though, are where this excess 40Ar* has come from, and whether it has been derived
from radioactive decay of 40K. One possibility is that the excess 40Ar* can be accounted for by radioactive decay
during long-term residence of magmas in chambers before eruption. Esser, McIntosh, Heizler, and Kyle
discounted this option for the Mt. Erebus anorthoclase phenocrysts.175 Dalrymple found that whereas the Mt.
Lassen (1915) plagioclase phenocrysts yielded excess 40Ar* and an anomalous K-Ar model “age,” a plagioclase
from the 1964 eruption of Surtsey only had argon whose isotopic composition matched that of air.176 Because
phenocrysts usually crystallize from lavas after eruption, they may arbitrarily trap excess 40Ar* during lava
cooling, 40Ar* that will thus not be from in situ40K radioactive decay.

Negative K-Ar Model “Ages” and Atmospheric Argon

Another relevant consideration bearing on these issues is the observation noted by Dalrymple that some
modern lava samples actually yield negative K-Ar model “ages,” apparently due to excess 36Ar177. Air has an
40Ar/36Ar ratio of 295.5, but some of Dalrymple’s samples had ratios less than 295.5 (and hence negative “ages”).
Some of the Mt. Ngauruhoe samples in this study also yielded 40Ar/36Ar ratios less than 295.5 (see table 4).
According to the straightforward interpretation of the K-Ar dating methodology, this should be impossible.
Dalrymple was not willing to attribute these anomalous ratios to experimental error, and neither was Richard
Reesman of Geochron Laboratories.

Dalrymple suggested three possible explanations that might account for the excess 36Ar178:

incorporation of “primitive argon,”

production of 36Ar by the radioactive decay of 36Cl, or

fractionation of atmospheric argon by diffusion.

He rejected the possibility of significant 36Ar formation in situ from nuclear reactions (option 2) because the
Cl content of basalts and the production rate of 36Cl by cosmic-ray neutrons both are too low to account for any
significant amount of 36Ar. Instead, Dalrymple seemed to favor option 3, that when atmospheric argon diffused
back into lavas as they cooled, 36Ar diffused in preferentially. However, he also recognized the weakness of
this argument—it is difficult to explain why some lavas are enriched in 36Ar while others are not (as at Mt.
Ngauruhoe also). To be consistent, if fractionation of atmospheric argon occurred during diffusion, then this
would mean that even supposedly “zero age” lavas actually have an apparent age, and that most lavas do not
degas upon eruption. In fact, depending on how strong the fractionation of 36Ar was during diffusion, it could
even be that all lavas do not completely degas.

This only leaves Dalrymple’s option 1, that the lavas with the anomalously high 36Ar come from areas of the
mantle (and perhaps also the crust) which have primordial argon that has not been diluted with radiogenic
40Ar and have not completely degassed. However, this means that there is no reason to assume that lavas
whose argon matches that in the atmosphere have degassed either, because they may have simply started
with argon which matches atmospheric argon. Nevertheless, Dalrymple is convinced that “much of the volatile
juvenile content may still be present in volcanic rocks quenched on the ocean floor.”179 Indeed, Dalrymple has
specifically defined excess 40Ar* as 40Ar that is not attributed to atmospheric argon or in situ radioactive decay
of 40K.180 Krummenacher is more cautious, attributing anomalous 40Ar/36Ar ratios and excess 40Ar* to the
“mass fractionation effect on argon of atmospheric isotopic composition” trapped in the lavas, as well as to the
presence of “magmatic” argon different in isotopic composition.181

The Role of Xenoliths

Is the excess 40Ar* simply “magmatic” argon, that is, argon that collects in the magma and then is inherited
by the lavas from it? Funkhouser and Naughton found that the excess 40Ar* in the 1800–1801 Hualalai flow,
Hawaii, resided in fluid and gaseous inclusions in olivine, plagioclase, and pyroxene in ultramafic xenoliths in
the basalt.182 The quantities of excess 40Ar* were sufficient to yield K-Ar model “ages” from 2.6 Ma to 2960 Ma.
However, Dalrymple subsequently only used the presence of the ultramafic xenoliths and their excess 40Ar*
contained in inclusions to explain partly the excess 40Ar* and anomalous K-Ar model “ages” he obtained
from the same 1800–1801 Hualalai flow, suggesting instead that the large single inclusions are not directly
responsible for the excess argon in the flows and that the 40Ar* is distributed more uniformly throughout the
rocks.183 Nevertheless, those K-Ar and Ar-Ar geochronologists who are concerned about the excess 40Ar* in their
samples undermining their “dating” are careful to check for xenoliths and xenocrysts. Esser, et al., did so and
discounted xenocrystic contamination.184

Xenoliths are present in the Ngauruhoe andesite flows (table 3), but they are minor and less significant as
the location of the excess 40Ar* residing in these flows than the plagioclase and pyroxene phenocrysts and the
much larger glomerocrysts of plagioclase, pyroxene, or plagioclase and pyroxene that predominate. The latter
are probably the early-formed phenocrysts that accumulated together in the magma within its chamber prior
to eruption of the lava flows. Nevertheless, any excess 40Ar* they might contain had to have been supplied to
the magma from its source. The xenoliths that are in the andesite flows have been described by Steiner as
gneissic and are therefore of crustal origin, presumably from the basement rocks through which the magma
passed on its way to eruption.185

Noble Gases from the Mantle

With the advent of the necessary technology, the isotopic concentrations and ratios of noble gases (including
argon) in rock and mineral samples are now obtainable. Honda, et al., have reported such analyses on
submarine basalt glass samples from Loihi Seamount and Kilauea, Hawaii, and concluded that helium and
neon isotopic ratios in particular, being uniquely different from atmospheric isotopic ratios, are indicative of
the mantle source area of the plume responsible for the Hawaiian volcanism rather than from atmospheric
contamination of the magma.186, 187 The 40Ar/36Ar ratios are consistent with excess 40Ar* having also come with
the magma from the mantle. A subsequent study, in which a larger suite of basalt glass samples, and also
samples of olivine phenocrysts, from the same and additional Hawaiian volcanoes were analyzed, concluded
that the isotopic systematics indicate that the helium and neon have been derived from the mantle and have not
been preferentially affected by secondary processes.188 Consequently, the excess 40Ar* also in these samples would
have been also carried from the upper mantle source area of these basalts by the magma plume responsible
for the volcanism. Moreira, Kunz, and Allègre have suggested, based on new experimental data from single
vesicles in mid-ocean ridge basalt samples dredged from the North Atlantic, that the excess 40Ar* in the upper
mantle may be almost double previous estimates (that is, almost 150 times more than the atmospheric
content relative to 36Ar), and represents a primordial mantle component not yet outgassed.189, 190 Burnard, Graham,
and Turner obtained similar results on the same samples, but maintained that because some of the 36Ar is
probably surface-adsorbed atmospheric argon, the upper mantle content of excess 40Ar* could be even ten times
higher.191

Similar results have been obtained from ultramafic mantle xenoliths in basalts from the Kerguelen
Archipelago in the southern Indian Ocean, and the considerable excess 40Ar* measured concluded to be a part
of the mantle source signature of this hotspot volcanism.192 However, it has not only been the suboceanic mantle
that has thus been sampled for its excess 40Ar* via such magma plumes. Matsumoto, Honda, McDougall,
and O’Reilly have reported high 40Ar/36Ar ratios in spinel-lherzolites from five eruption centers in the
youthful (<7 Ma) Newer Volcanics of southeastern Australia.193 These anhydrous lherzolites have compositions
representative of the upper lithospheric mantle, and the significant excess 40Ar* in them indicates the presence
of a subcontinental mantle reservoir with a very high 40Ar/36Ar ratio, and thus substantial excess 40Ar*, similar
to that found in mid-ocean ridge and plume/hotspot basalts. Another example is the Cardenas Basalt and
associated diabase (Middle Proterozoic) of eastern Grand Canyon, regarded as part of the pervasive mafic
mid-continental magmatism of the southwestern United States and thus also sourced from the subcontinental
mantle. Austin and Snelling have found that the 40Ar/36Ar–40K/36Ar isochrons for fourteen and six samples
of these rocks, respectively, yield initial 40Ar/36Ar ratios of 787±118 and 453±42, indicative of some initial excess
40Ar*.194

Sampling the Mantle with Diamonds and Their Inclusions

Another means of “sampling” the mantle is the study of diamonds and their micro-inclusions. It is now firmly
established that diamonds are thermodynamically stable in the pressure-temperature regime in the mantle at
depths greater than 150 km, and their origin is believed to extend back to the Archean and the early crust of
the Earth.195, 196 Diamonds are formed in a number of processes associated with two rock types, ecologite and
peridotite, xenoliths of which are also brought up into the upper crust with diamonds from the upper mantle
below continental Precambrian shields (cratons) by kimberlite and lamproite “pipe” eruptions.197, 198, 199 Even
though the host kimberlite or lamproite may be relatively young (even in conventional terms), many diamonds
date back to the Archean and thus the early history of the Earth.200, 201 To account for all this evidence, it is
postulated that the formation of most diamonds was closely associated with subduction of the Archean oceanic
crust into the mantle, the required carbon (which was originally thought to be primordial carbon already
in the mantle) now believed to derive from sedimentary marine carbonates and biogenic carbon from bacteria/
algae in the sediments subducted with the oceanic crust.202, 203, 204, 205, 206

The noble gas contents of diamonds are consistent with their ancient and mantle origin, high helium isotopic
ratios (290 times the atmospheric ratio) being regarded as primordial and rivaling those measured for the sun
today.207, 208 Of significance here is the postulation that He, Ar, K, Pb, Th, and U are added to the convecting
upper mantle circulation, and the proportions and isotopic compositions are strongly determined by entrainment
from the lower mantle (below 670 km).209, 210 This is reflected in those Ar isotopic measurements that have been
made on diamonds and their micro-inclusions.

Rather than focus on attempting to date only diamond micro-inclusions as others had done, Zashu, Ozima,
and Nitoh carefully selected ten Zaire diamonds and examined them for purity before undertaking K-Ar
dating analyses of the diamonds themselves.211 However, at the outset they noted that there had been almost
no direct radiometric dating of diamonds except for conventional K-Ar dating, and the results had been
questionable due to the possible presence of excess 40Ar*. To avoid this problem, they used the K-Ar isochron
dating method. Their experimental data showed good linear correlations, but these isochrons yielded an age
of 6.0±0.3 Ga, which of course was unacceptable because these diamonds would be older than the earth itself.
Mistakes in the experimental procedure were easily discounted, so they were forced to conclude that excess
40Ar* was responsible, and that it needed to be in a fluid state to ensure the homogenization necessary to
give such a constant 40Ar/K ratio. Alternately, they speculated that the diamonds might differ in K isotopic
composition from common potassium, but this was discounted in a follow-up study in which it was found
that 40K was present in these diamonds in normal abundance.212 Because 40Ar/39Ar analyses yielded the same
unacceptable “age,” it was concluded that the excess 40Ar* was not generated in situ, but was an inherited or
“trapped” component from the mantle reservoir when and where the diamonds formed.

These Zaire diamonds are not the only ones which have yielded excess 40Ar*. Phillips, Onstott, and Harris
used a laser-probe to 40Ar/39Ar date eclogitic clinopyroxene inclusions in diamonds from the Premier kimberlite,
South Africa, and found moderate 40Ar/36Ar ratios indicative of much less excess 40Ar* than in the Zaire
diamonds.213 The “age” of these eclogitic diamonds was thus determined to be 1.198±0.014 Ga, much younger than
the 3.3 Ga peridotitic diamonds at Kimberley and Finsch, also in South Africa, so Phillips, et al., interpreted
the moderate excess 40Ar* as characteristic of mantle conditions prevailing at the time and in the region of
Premier eclogitic diamond formation.214

Zashu, et al., postulated that the excess 40Ar* in the Zaire diamonds needed to be in a fluid state.215 Though
Navon, Hulcheon, Rossman, and Wasserburg did not analyze for argon when they investigated fluids in
micro-inclusions in diamonds from Zaire and Botswana, they found a high content of volatiles and incompatible
elements in the uniform average composition of the micro-inclusions, with the amounts of water and CO2 (in
carbonates) almost an order of magnitude higher than the volatile contents of kimberlites and lamproites (host
rocks to diamonds).216 At 13 wt%, the chlorine levels were also much higher than those of kimberlites (<0.1%),
although the bulk composition of the micro-inclusions, including the high K2O content (up to 29.7 wt%),
resembled that of such potassic magmas. They concluded that these micro-inclusions represent the volatile-rich
(~40% volatiles) fluid or melt from the upper mantle in which the diamonds grew, and that because of the high
volatile content in this hydrous mantle fluid, high levels of rare gases may also be expected and explain the high
40Ar/K ratios (the excess 40Ar*) and anomalous “ages.”

As a result of continued investigation of the Zaire cubic diamonds, which produced 40Ar/39Ar age spectra
yielding a ~5.7 Ga isochron, Ozima, Zashu, Takigami, and Turner discovered that just as there was an
excellent correlation between their potassium contents and 40Ar/36Ar ratios, there is also a correlation between
their chlorine contents and 40Ar.217 They concluded from their data

that the 40Ar is an excess component which has no age significance, and that the 40Ar and its associated
potassium are contained in sub-micrometer inclusions of mantle-derived fluid.

Turner, Burgess, and Bannon also used the 40Ar/39Ar technique through correlations with K, Cl, and 36Ar
to unscramble the mixtures of radiogenic and parentless (excess) Ar components in fluid inclusions in “coated”
Zaire diamonds and in olivine from an East African mantle xenolith.218 Their results proved conclusively that 40Ar
is present in a widespread chlorine-rich component, which implies the existence of H2O/CO2-rich phases with
40Ar/Cl ratios that are “remarkably uniform over large distances,” with enrichments of these two incompatible
elements by almost four orders of magnitude relative to bulk upper-mantle values. Clearly, excess 40Ar* is
abundant in the mantle and can be easily transported up into the crust.

Crustal Excess 40Ar*

Is there only evidence for excess 40Ar* in the mantle, gleaned from rocks (basalts and ultramafic xenoliths)
and minerals (olivine, pyroxene, plagioclase, and diamonds) that were formed in, or ascended from, the mantle?
Patterson, et al., envisage noble gases from the mantle (and the atmosphere) migrating and circulating through
the crust, so there should be evidence of excess 40Ar* in crustal rocks and minerals.219 In fact, noble gases in CO2-rich natural gas wells support such migration and circulation—that is, the isotopic signatures clearly indicate
a mantle origin for the noble gases, including amounts of excess 40Ar* in some CO2-rich natural gas wells
exceeding those in the mantle-derived mid-ocean ridge basalts.220, 221, 222, 223, 224 Staudacher also notes that the quantities
of excess 40Ar* in the continental crust can be as much as five times that found in such mantle-derived midocean
ridge basalts, strongly suggesting that excess 40Ar* in crustal rocks and their constituent minerals
could well be the norm rather than the exception, thus making all K-Ar (and Ar-Ar) dating questionable.225

It has now been established that some diamonds can form in the crust during high-grade metamorphism and via shock metamorphism during meteorite or asteroid impact.226, 227, 228 The pressures and temperatures of high-grade
metamorphism had been regarded as insufficient to produce diamonds, but the key ingredient was found
to be volatile N2-CO2-rich fluids. Noble gas data on these diamonds are not yet available, due to their size
and rarity, but such data have been definitive in establishing the crustal origin of carbonado diamonds.229
Nevertheless, they still contain excess 40Ar*.

Dalrymple, referring to metamorphism and anatexis of rocks in the crust, commented, “If the rock is
heated or melted at some later time, then some or all the 40Ar may escape and the K-Ar clock is partially or
totally reset.”230 In other words, 40Ar* escapes to migrate in the crust where it may then be incorporated in other
minerals as excess 40Ar*, just as 40Ar* degassing from the mantle does. Thus, for example, excess 40Ar* has
been recorded in many minerals (some of which contain no 40K) in crustal rocks, such as quartz, plagioclase,
pyroxene, hornblende, biotite, olivine, beryl, cordierite, tourmaline, albite, and spodumene in pegmatites,
metamorphic rocks, and lavas.231, 232 And it is not just K-Ar dating analyses that detect excess 40Ar*, as Lanphere
and Dalrymple used the 40Ar/39Ar method to confirm the presence of excess 40Ar* in feldspars and pyroxenes.233
Indeed, in a recent study, 128 Ar isotopic analyses were obtained from ten profiles across biotite grains in
amphibolite-granulite facies metamorphic rocks, and apparent 40Ar/39Ar “ages” within individual grains ranged
from 161 to 514 Ma.234 The investigators concluded that these observations cannot be solely due to radiogenic
build-up of 40Ar*, but must be the result of incorporation by diffusion of excess 40Ar* from an external source,
namely, 40Ar* from the mantle and other crustal rocks and minerals. Indeed, Harrison and McDougall were
able to calculate a well-defined law for 40Ar diffusion from hornblende in a gabbro due to heating.235 They also
found that the excess 40Ar* which had developed locally in the intergranular regions of the host gabbro reached
partial pressures in some places of at least 10-2 atm.

This crustal migration of 40Ar* is known to cause grave problems in attempted regional geochronology
studies. In the Middle Proterozoic Musgrave Block of northern South Australia, Webb found a wide scatter
of K-Ar mineral ages ranging from 343 Ma to 4493 Ma due to inherited (or excess) 40Ar*, so that no meaningful
interpretation could be drawn from the rocks (granulite, gneiss, pseudotrachylite, migmatite, granite, and
diabase).236 Of the diabase dikes which gave anomalous ages, he concluded that

The basic magmas probably formed in or passed through zones containing a high partial pressure of 40Ar*,
permitting inclusion of some of the gas in the crystallizing minerals.

Likewise, when Baski and Wilson attempted to argon date Proterozoic granulite-facies rocks in the Fraser
Range (Western Australia) and Strangways Range (central Australia), they found that garnet, sapphirine, and
quartz in those rocks contained excess 40Ar* that rendered their argon dating useless because of “ages” higher
than expected.237 They also concluded that the excess 40Ar* was probably incorporated at the time of formation of
the minerals, and their calculations suggested a partial pressure of ~0.1 atm Ar in the Proterozoic lower crust
of Australia, which extends over half the continent.

In a detailed 40Ar/39Ar dating study of high-grade metamorphic rocks in the Broken Hill region of New South
Wales (Australia), Harrison and McDougall found evidence of widely distributed excess 40Ar*.238 The minerals
most affected were plagioclase and hornblende, with step heating 40Ar/39Ar “age” spectra yielding results of up
to 9.588 Ga. Such unacceptable “ages” were produced by excess 40Ar* release, usually at temperatures of 350–650°C and/or 930–1380°C, suggesting the excess 40Ar* is held in sites within the respective mineral lattices
with different heating requirements for its release. There are three principal trapping sites for Ar in solids: structural holes, edge dislocations, and lattice vacancies. (Argon is also known to be held sometimes in some
minerals in fluid inclusions.) Clearly, this study shows that at crustal temperatures, which are less than 930°C,
some excess 40Ar* will always be retained in those trapping sites in minerals where it is obviously “held” more
tightly, thus rendering K-Ar and 40Ar/39Ar dating questionable. Harrison and McDougall were only able to
produce a viable interpretation of the data because they made assumptions about the expected age of the rocks
and of a presumed subsequent heating event (based on Pb-Pb and Rb-Sr dating), the latter being the time when
they conjecture that accumulated 40Ar* was released from minerals causing a significant regional Ar partial
pressure of ~3x10-4 atm to develop.239

Mantle-Crust Domains and Excess 40Ar*

Harte and Hawkesworth have identified domains within the mantle and crust and described the interaction
between them, all of which is relevant to the migration and circulation of argon (and thus excess 40Ar*) from
the lower mantle to the crust and to lavas extruded on the earth’s surface.240 The six domains are physically
distinct units which show wide differences in average physical and chemical properties, as well as apparent
age, structure, and tectonic behavior. They are the lower mantle (below 670 km), upper mantle, continental
mantle lithosphere, oceanic mantle lithosphere, continental crust, and oceanic crust, and each is a distinct
geochemical reservoir. Each domain may provide material for magmatic rocks, and particular geochemical
features of magmas may be associated with particular domains. Thus the convecting upper mantle which
comes to the surface at mid-ocean ridges may be identified as the source of most geochemical features of midocean
ridge basalts, including their excess 40Ar* content. Similarly, the convecting lower mantle is regarded
as the primordial or bulk earth geochemical reservoir, which may also contribute excess 40Ar* to mid-ocean
ridge basalts, but is more important for its contribution to ocean island basalts (for example, Hawaii) and
other plume-related basalts (continental alkali basalts and continental flood basalts). However, considerable
complexity may be added to the deeper mantle geochemical structure as a result of localized accumulation of
subducted oceanic lithosphere.

Porcelli and Wasserburg have proposed a steady-state upper mantle model for mass transfer of rare gases,
including argon.241 The rare gases in the upper mantle are derived from mixing of rare gases from the lower
mantle, subducted rare gases, and radiogenic nuclides produced in situ. Porcelli and Wasserburg claim that all
of the 40Ar in the closed-system lower mantle has been produced by 40K decay in the lower mantle, but this claim
is based on the assumption of a 4.5 Ga earth. In any case, they contradict themselves, because they also state,
“The lower mantle is assumed to have evolved isotopically approximately as a closed system with the in situ
decay of 129I, 244Pu, 238U, 232Th, and 40K adding to the complement of initial rare gases.”242 In other words, they
admit that some of the 40Ar must be primordial and not derived from radioactive 40K. They then go on to claim
that in the upper mantle, 40K decay further increases the radiogenic 40Ar from the lower mantle by a factor of
~3, but again this presupposes a 4.5 Ga earth and doesn’t allow for primordial 40Ar that could well also be in the
upper mantle if it is admitted to be in the lower mantle.

In the case of the continental and oceanic lithospheric domains, the lack of convective stirring means that
different geological processes and events may implant in each domain a variety of geochemically distinct
materials that will remain isolated from one another. Therefore, these domains do not have a single set of
geochemical characteristics; thus identification of geochemically defined “sources” with particular physically
defined crust-mantle domains is complex, and the geochemical definition of particular reservoirs cannot be
regarded as simply definition of major physical entities. Nevertheless, excess 40Ar* will be added to these
domains by the passage of basaltic magma plumes from the upper mantle to the earth’s surface.

Furthermore, the processes of oceanic lithosphere formation from the convecting upper mantle in association
with mid-ocean ridge activity mean that its isotopic characteristics everywhere will be largely similar to those
of the convecting upper mantle and mid-ocean ridge basalts, including the addition of excess 40Ar*. The corollary
to this is that the oceanic crust is formed as part of these same processes. However, the oceanic crust generally
has a thin veneer of sediments over it, and thick wedges of sediments adjacent to the domains of continental
crust, whereas sections of oceanic crust are hydrothermally altered. The compositions of these components of
the oceanic crust may, therefore, include a considerable contribution from continental detritus and ocean water,
so that this oceanic crustal material may give rise to a distinct geochemical reservoir, the fate of which during
subduction back into the upper mantle becomes critically important if it contributes to island arc volcanics,
plume-related intra-place magmas, and mantle-derived xenoliths.

The complexity of continental crustal material is well known through direct observation, and the mantle
lithosphere attached to it may be expected to show a similar complexity. Nevertheless, it is evident that excess
40Ar* also resides in the continental mantle lithosphere, as indicated by xenoliths.243 Likewise, there is evidence of
excess 40Ar* in crustal magmatic rocks (for example, gabbros, pegmatites, migrating through metamorphic
terrains, and in natural gas in sedimentary reservoirs.244, 245, 246, 247, 248, 249

Mt. Ngauruhoe in its Tectonic Framework

The presence, therefore, of excess 40Ar* in the recent andesite flows at Mt. Ngauruhoe is to be expected.
The Taupo Volcanic Zone is a volcanic arc and marginal basin of the Taupo-Hikurangi arc-trench system
(see fig. 1 again), which is a southward extension of the Tonga-Kermadec arc into the continental crustal
environment of New Zealand’s North Island.250 Geophysical investigations indicate that the Pacific Plate is
being obliquely subducted beneath the Australian Plate on which most of New Zealand’s North Island sits,
and that the volcanoes of the Taupo Volcanic Zone, including Ngauruhoe in the Tongariro Volcanic Center, are
about 80 km directly above the subducting Pacific Plate, a zone of earthquakes revealing where the movement
is taking place.251 Friction along the plane of contact is believed to cause melting to produce pockets of magma,
which then feed via conduits to the volcanoes above. Thus the recent andesite flows at Mt. Ngauruhoe are calcalkaline
island arc volcanics.

The tectonic and geochemical framework of the Ngauruhoe andesite flows within the mantle-crust domains
of Harte and Hawkesworth is that of subducting oceanic crust (derived from the convecting upper mantle),
carrying with it the wedge of continental sedimentary detritus which has accumulated at the continental
margin and in the adjacent trench to the east of the coastline.252 Attached beneath the subducting oceanic
crust is its associated oceanic mantle lithosphere, and together they are being thrust downwards into the
upper mantle. Above the subducting plate are the continental crust and continental mantle lithosphere of the
overriding plate, the continental crust being at the contact plane at shallow depths near the trench, and then the
attached continental mantle lithosphere beneath at a depth of about 35 km.253 Thus the geochemical reservoir
from which the Ngauruhoe andesite magma has been drawn is potentially a mixture of melted oceanic crust,
continental sedimentary detritus and continental crust, and possibly continental mantle lithosphere, or even
upper mantle.

Genesis of the Mt. Ngauruhoe Andesite Magma and its Excess 40Ar*

One of the easier investigations of the petrogenesis of these volcanic rocks of the Taupo Volcanic Zone was that of
Stipp and of Ewart and Stipp.254, 255 They analyzed samples that had been systematically collected, including not only
the lavas and the pyroclastics, but also the Permian to Jurassic interbedded greywackes, siltstones, and shales
(the potential crustal source rocks), which are spatially related to, and underlie, the volcanics. Of primary interest
were Sr, Rb, and K contents, and 87Sr/86Sr and 87Rb/86Sr ratios. Three possibilities for the origin of the calc-alkaline
andesite magma were under investigation: fractional crystallization of a basalt magma under oxidizing conditions;
some form of hybridization between basaltic and acidic magmas, possibly followed by fractional crystallization;
and derivation of a primary andesite magma from the upper mantle. Ewart and Stipp regarded their Sr isotopic
data as more consistent with the production of the andesites by partial assimilation of sedimentary material by
basaltic magma (derived from the upper mantle), the adjacent greywackes, siltstones, and shales being the most
likely sedimentary material, and the unassimilated gneissic xenoliths probably representing the basement rocks
to those sediments.256 However, they admitted that the data did not exclude the possibility of a primary andesitic
magma derived directly from the upper mantle, provided that some assimilation of crustal material modified it
prior to eruption.

Subsequent investigations by Cole favored the alternate petrogenetic model of a primary andesitic magma.257 He
suggested that the subducting oceanic crust assimilated the greywacke-siltstone-shale and overlying sediments
east of the Taupo Volcanic Zone to produce amphibolite, which subsequently broke down to produce phlogopite
eclogite below 90 km. This in turn partially melted at 150–200 km, and the resultant magma fractionated
in the upper mantle or lower crust to produce andesite. However, based on rare-earth element geochemistry,
Cole, Cashman, and Rankin modified that petrogenetic model, suggesting that while the andesite magma
genesis was probably associated in the upper mantle with the downgoing slab and some crustal contamination
occurred, the andesite does not appear to have had an eclogite parent.258 This would then suggest that the melting
associated with the subducting slab to generate the andesite magma occurred at a depth of less than 90 km.

Graham and Hackett agreed with this conclusion, demonstrating from geophysical evidence that the top
of the subducting slab is at a depth of about 80 km below Ngauruhoe, and that the crust there is probably less
than 20 km thick.259 Thus the upper mantle wedge between would consist only of plagioclase-peridotite and spinelperidotite.
At 80 km depth the hydrated amphibolite assemblage of the upper portion of the subducting slab
of oceanic crust and oceanic mantle lithosphere would have started to dehydrate, thus liberating water and
possibly other volatile constituents into the overlying upper mantle wedge, significantly lowering its melting
point. Graham and Hackett then showed that the geochemical evidence requires the andesite magma for the
Ngauruhoe lava flows to have been generated from an original low-alumina basalt magma produced in the upper
mantle wedge by anatexis of the asthenosphere (uppermost mantle) and/or subcontinental mantle lithosphere
probably catalyzed by hydrous, metasomatic fluids from the subducting slab.

Some specific geochemical enrichment then appears to have occurred as a result of this mantle metasomatism
and continental crustal contamination during ascent and storage of the magma. Graham and Hackett used
least squares geochemical modeling to show how the andesite magma could be generated from such a parent
basalt magma by a process of combined assimilation of crustal material (addition of 6% assimilant) and fractional
crystallization (30% removal of crystals).260 Furthermore, the presence of xenoliths in the Ngauruhoe andesite
flows, particularly the vitrified meta-greywacke and gneissic xenoliths, indicate conclusively that the assimilant
was most likely a partial melt of gneiss, originally the adjacent greywacke-siltstone-shale sediments.261, 262

These processes responsible for the generation of the andesite magma did not diminish the excess 40Ar*
content of the resultant flows. Though the amount of excess 40Ar* is not high when compared with that found
in mid-ocean ridge basalts, it is nonetheless significant that the excess 40Ar* was still present in the lavas upon
eruption and cooling. The evidence indicates that the parent basaltic magma was generated in the upper mantle
where the excess 40Ar* in the geochemical reservoir is now known to be upwards of 150 times more than the
atmospheric content, relative to 36Ar. The subsequent crustal contamination and fractional crystallization to
form the andesite magma during ascent, and the degassing of the magma during eruption and lava flow and
cooling, did not remove all the excess 40Ar*, a small portion of which was left to be trapped in the congealed lava
and its constituent minerals.

This model for the generation of the andesite magma in the post-Flood world is, of course, based on the plate
tectonics model for global tectonics through earth history. Even though the postulated plate movements today
are extremely slow, and thus extrapolated back over millions of years by uniformitarians, a catastrophic model
for plate tectonics in the context of the Flood is entirely compatible with both Scripture and the scientific data.263
Plate movements are regarded as occurring catastrophically during the Flood and then rapidly slowing down to
today’s rates in the post-Flood era.

Conclusions

The fact that there is even some excess 40Ar* in these recent andesite flows, and that it appears to have
ultimately come from the upper mantle geochemical reservoir—where it is regarded as leftover primordial argon
not yet fully expelled by the process of outgassing that is supposed to have occurred since the initial formation
of the earth—has very significant implications.

First, this is clearly consistent with a young earth, where the very short time scale since the creation of
the earth has been insufficient for all the primordial argon to be released yet from the earth’s deep interior.
Furthermore, it would also seem that even the year-long global catastrophic Flood, when large-scale convection
and turdecer occurred in the mantle, was insufficient to expel all the deep earth’s primordial argon.264

Second, this primordial argon is, in part, “excess” 40Ar not generated by radioactive decay of 40K, which has
then been circulated up into crustal rocks where it may continue migrating and building up to partial pressure
status regionally. Because the evidence clearly points to this being the case, when samples of crustal
rocks are analyzed for K-Ar “dating” the investigators can never really be sure that whatever 40Ar* is in the
samples is from in situ radioactive decay of 40K since the formation of the rocks, or whether some or all of it is
from the “excess 40Ar*” geochemical reservoirs in the lower and upper mantles. This could even be the case
when the K-Ar analyses yield “dates” compatible with other radioisotopic “dating” systems and/or with fossil
“dating” based on evolutionary assumptions. And there would be no way of knowing because the 40Ar* from
radioactive decay of 40K cannot be distinguished analytically from primordial 40Ar that is not from radioactive decay,
except, of course, by external assumptions about the ages of the samples.

Therefore, these considerations call into question all K-Ar “dating,” whether “model ages” or “isochron ages,”
and all 40Ar/39Ar “dating,” as well as “fossil dating” that has been calibrated against K-Ar “dates.” Although
seemingly insignificant in themselves, the anomalous K-Ar “model ages” for these recent andesite flows at
Mt. Ngauruhoe, New Zealand, lead to deeper questions. Why is there excess40Ar* in these rocks? From where
did it come? Answers to these questions in turn point to significant implications that totally undermine such
radioactive “dating” and that are instead compatible with a young earth.

Future Research

Further research is very definitely warranted. The most pressing need is to attempt to quantify how much
primordial 40Ar there is today in the upper mantle, as well as how much has circulated into crustal rocks, how much
is in natural gas reservoirs, and how much might have escaped into the atmosphere during 6,000–7,000 years (including at accelerated rates during the Flood). It might then be possible to quantify how much primordial 40Ar
there was in the mantle at the time of the earth’s creation. From these calculations and associated modeling
exercises there might develop quantifiable evidence for the earth’s youth.

Additionally, further research is needed to quantify how much “excess 40Ar*” is in all the crustal rocks and
minerals that have been, and are, subject to K-Ar and 40Ar/39Ar “dating.” This would include what are regarded as
mantle xenoliths and xenocrysts (for example, diamonds). It is helpful to show on the one hand that such “dating”
is questionable, but on the other hand there are still many “dates” that are concordant, that is, they agree with
other uniformitarian dating systems and schemes. So ultimately we need to explain this agreement when
other “dates” are discordant and anomalous. There may, in fact, be some pattern or systematic way in which
“excess 40Ar” has been trapped in rocks and occluded in minerals at different levels (depths and relative ages) in
the geological record. If so, then K-Ar and 40Ar/39Ar “dating” would irrevocably be discredited.

Steiner, A., 1958. Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths. New Zealand Journal of Geology
and Geophysics 1:325–363.

Clark, Ref. 70.

Hackett and Houghton, Ref. 9.

Steiner, Ref. 75.

Clark, Ref. 70.

Ewart, A. and J. J. Stipp, 1968. Petrogenesis of the volcanic rocks of the central North Island, New Zealand, as indicated by
a study of Sr87/Sr86 ratios, and Sr, Rb, K, U and Th abundances. Geochimica et Cosmochimica Acta 32:699–736.

Graham and Hackett, Ref. 1.

Cole, et al., Ref. 8.

Nairn and Wood, Ref. 11.

Nairn, et al., Ref. 57.

Hackett and Houghton, Ref. 9.

Nairn, et al., Ref. 57.

Hackett and Houghton, Ref. 9.

Hackett and Houghton, Ref. 9.

Nairn, et al., Ref. 57.

Clark, Ref. 70.

Cole, J. W., 1978. Andesites of the Tongariro Volcanic Centre, North Island, New Zealand. Journal of Volcanology and
Geothermal Research 3:121–153.

Harrison, T. M. and I. McDougall, 1981. Excess 40Ar in metamorphic rocks from Broken Hill, New South Wales: Implications
for 40Ar/39Ar age spectra and the thermal history of the region. Earth and Planetary Science Letters 55:123–149.

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Answers in Genesis is an apologetics ministry, dedicated to helping Christians defend their faith and proclaim the gospel of Jesus Christ effectively. We focus on providing answers to questions about the Bible—particularly the book of Genesis—regarding key issues such as creation, evolution, science, and the age of the earth.